PROJECT SUMMARY The gut microbiome is associated with the pathogenesis of a diverse number of human neurobehavioral disorders including autism spectrum disorders, major depression, and Parkinson?s disease. Murine models of these diseases suggest that microbiome-neuroimmune interactions can mediate behavioral and physiological abnormalities, specifically through changes in brain transcriptome, altered microglial maturation and function, and altered integrity of the blood brain barrier. However, it is unclear how specific bacterial functional pathways, or bacterial byproducts, affect neurobehavioral processes. Better tools are necessary to functionally manipulate the gut microbiome and attain a better mechanistic understanding of the microbiome-gut-brain relationship. We have devised a new technique to ?knock-in? functions into the gut microbiome of conventionally- raised wild-type mice to investigate their effects on host physiology. While investigating the role of bacterial bile acid deconjugation on host metabolism, we noticed tremendous effects on host behavior and cognition. Previous studies have linked bile acids to host neuroinflammation and have suggested that bile acids could potentially affect cognition. Hence, we hypothesized that bile acids are key mediators of the microbiome- gut-brain axis. This exploratory, two-year grant will help us establish the collaborative team and resources to investigate this hypothesis using our novel technique in a high-fat diet (HFD)/obesity-induced mouse model of neuroinflammation and behavior dysfunction. Mice consuming a HFD for greater than 20 weeks have increased proinflammatory cytokines, gliosis, and alterations in the brain vasculature, as well as, memory dysfunction and increased anxiety. Our preliminary results show that increased bacterial bile acid deconjugation protects against HFD-induced memory dysfunction despite not affecting host adiposity or metabolism. Hence, in our first specific aim, we will assess whether bacterial bile acid deconjugation affects host cognition and anxiety by using a battery of behavioral tests. This will help us determine whether our preliminary results are the result of true cognitive changes or the result of other potential metabolic or physiological processes. Furthermore, previous studies show that bile acids can have profound effects on microglial activation. We will evaluate whether increased bacterial bile acid deconjugation can affect neuroinflammation by assessing microglia transcriptional activity, microglia activation, and cytokine quantification. By pursuing the experiments in our specific aims, we will be able to ascertain whether bile acids are the means through which the gut microbiome can affect the host cognition and neuroinflammation, and attain enough data and understanding of this relationship to develop more sophisticated research programs, which will lay the foundation for a better understanding of how the gut microbiome contributes to neuropathology and how it can be manipulated to treat recalcitrant diseases.